Do I just let it go knowing my gambling money is gone forever?

Yessss! Yes yes, and yes. That fact that you are at least thinking that is a step in the right direction. There are support groups that can help with that process as well such as gamblers anonymous. By letting it go it helps you move on, if you don't the thought of more gambling will remain strong and could seem like a good option. Get support so that part of your brain does not give you "crazy" ideas.
Beats me. Whether you "let it go" is up to you, and only you. Many and probably most gamblers may reach a point where they have blown all their money, but that rarely stops gambling. As soon as the person accumulates more resources (through work, begging, borrowing or stealing) they gamble again. If you have a dependency, it may require getting help. Gamblers anonymous and counseling can help.